A lighting device for vehicles is known from DE 10 2008 048 765 A1, having a flat optical waveguide with opposing flat sides, as well as a plurality of light sources. The light sources are disposed on a light input side of the flat waveguide. The light sources are each allocated a planar light input surface at which the light emitted from the light source enters the flat waveguide. The entering light is subjected to total reflection on the opposing flat sides, and emitted at a light output side of the flat waveguide toward a light output surface designed as a narrow side. The light enters the flat waveguide, undirected, via the planar light input surface, such that brightness differences occur at the narrow light output surface. In particular in the main beam direction in front of the light source in the region of an optical axis thereof, relatively high brightness values are obtained, leading to a non-homogenous illumination of the light output surface.
A generic lighting device for vehicles, having a flat waveguide and a plurality of light sources is known from DE 10 2007 057 399 A1, in which the reflective light input surfaces allocated to the light sources are spherical, i.e. not planar. As a result, a punctiform bright illumination of a light output surface can be softened in the main beam direction directly in front of the light source. The photometric efficiency, however, is compromised, because light diffusion occurs due to the frequent total reflection of the input light at the opposing flat sides, which is then lost to the required light distribution of the lighting device.